1. Field of the Invention
The invention relates to a halftone-type phase-shift mask blank, a halftone-type phase-shift mask, and a method for manufacturing the same. Particularly, the invention relates to a halftone-type phase-shift mask suitable for use with an ArF excimer laser (193 nm) and an F2 excimer laser (157 nm), both serving as next-generation short-wavelength exposure light sources, and to a blank serving as a raw material of the phase-shift mask.
2. Description of the Related Art
A framework for mass-producing 256-Mbit DRAM is currently established. A further increase in integration from a megabit-class packing density to a gigabit-class packing density is about to be attempted. In association with such an increase in packing density, a rule for designing an integrated circuit has become much more stringent. Occurrence of a request for a fine pattern having a line width (half pitch) of 0.10 μm or less will surely arise.
An increase in resolution of a pattern has hitherto been pursued by rendering the wavelength of the exposure light source shorter as one means for rendering a pattern finer. As a result, a KrF excimer laser (248 nm) and an ArF excimer laser (193 nm) have primarily been used as the exposure light sources of the current photolithography method.
Shortening of an exposure wavelength results in an improvement in resolution but simultaneously causes a decrease in the depth of a focus, thereby posing adverse effects, such as an increase in the burden of designing an optical system including a lens or a decrease in the safety of processes.
In order to address those problems, a phase-shift method has become employed. The phase-shift method employs a phase-shift mask as a mask for transferring a fine pattern.
The phase-shift mask is formed from, e.g., a phase-shifter section to be used for forming a pattern on a mask and a non-pattern section where no phase shifter section is present. Phases of the light rays that have passed through the phase-shifter section and the non-pattern section are shifted from each other through 180°, thereby causing optical mutual interference in the boundaries of the pattern. Thus, contrast of a transferred image is improved. The amount of phase shift φ (rad) arising in the light that passes through the phase-shifter section has been known to depend on a complex refractive index real part “n” of the phase shifter section and a film thickness “d,” and the relationship defined in Equation (1) is known to stand.φ=2πd(n−1)/λ  (Eq. 1)
Here, λ designates the wavelength of exposure light. In order to cause a 180° phase shift, the film thickness “d” merely has to be set asd=λ/{2(n−1)}  (Eq. 2).By means of the phase-shift mask, focal depth to be used for achieving a required resolution is increased. The resolution and the applicability of processes can be simultaneously improved without changing the exposure wavelength.
Practically, the phase-shift mask can be roughly divided into a total-transmission-type (Levenson-type) phase-shift mask and a halftone-type phase-shift mask by means of an optical transmission characteristic of the phase shifter section forming a mask pattern. In the case of the former phase-shift mask, the phase shifter section is identical in optical transmissivity with the non-pattern section (a light transmission section). The mask is substantially transparent to the exposure wavelength. This phase-shift mask is generally said to be effective for transferring lines and spaces. In the case of the latter halftone-type phase-shift mask, the optical transmissivity of the phase shifter section (an optical semi-transmission section) is a few percent to a few tens of percent that of the non-pattern section (optical transmission section). The latter halftone-type phase-shift mask is said to be effective for forming contact holes and isolated patterns.
As shown in FIG. 14, the halftone-type phase shift mask has a transparent substrate 2 on which are formed at least a light transmission section 7 and a halftone phase shifter section 8 having a semi-optical transmission characteristic and a phase shift function. In terms of construction of the halftone phase shifter section 8, the halftone-type phase-shift mask can be roughly divided into a single-layer-type phase-shift mask and a multilayer-type phase-shift mask. The single-layer type halftone phase-shift mask is now the mainstream, because of ease of processing. In most single-layer-type halftone phase-shift masks, the halftone phase shifter section is formed from a single-layer film made of MoSiN or MoSiON. In contrast, in the case of a multilayer-type halftone phase-shift mask, the halftone phase-shifter section is primarily formed by combination of a layer for controlling transmissivity and a layer for controlling the amount of phase shift. The multi-layer-type halftone phase-shift mask enables independent control of a polarizing characteristic typified by transmissivity and independent control of amount of phase shift (phase angle).
In association with miniaturization of an LSI pattern, in the future the wavelength of the exposure light source (i.e., exposure wavelength) is expected to become shorter from the current KrF excimer layer (248 nm) to an ArF excimer laser (193 nm) and further to an F2 excimer laser (157 nm). As the wavelength of the exposure light source becomes shorter, the range of choices of materials of a halftone phase shifter section which satisfies a predetermined transmissivity and the amount of phase shift becomes narrower. In association with shortening of wavelength of the exposure light, material having high optical transmissivity is required, in view of a conventional wavelength. Consequently, there arises a problem of a decrease in etch selectivity to a quartz substrate, which would be caused at the time of processing of a pattern. The multilayer-type (two-layer film) halftone phase shifter has an advantage of easy selection of material which enables control of a phase difference and transmissivity by combination of two-layer films and an advantage of the ability to select a material which would play a role of an upper etch stopper (as described in JP-A-2001-174973). For these reasons, development of a multilayer-type (two-layer-type) halftone phase-shifter is pursued.
In association with miniaturization of an LSI pattern, in the future the wavelength of the exposure light source (i.e., exposure wavelength) is expected to become shorter from the current KrF excimer layer (248 nm) to an ArF excimer laser (193 nm) and further to an F2 excimer laser (157 nm). The film of the current dominant halftone-type phase-shift mask is designed such that a portion of the halftone phase-shifter section assumes an exposure light transmissivity of around 6%. With a view toward a higher resolution, a demand for a halftone-type phase-shift mask having a higher transmissivity is now emerging. A future requirement for a transmissivity of 15% or more is said to arise. In association with shortening of wavelength of the exposure light source and an increase in transmissivity, the range of choices of materials of a halftone phase shifter section which satisfy a predetermined transmissivity and the amount of phase shift becomes narrower. Moreover, another problem is that etch selectivity to a quartz substrate becomes smaller at the time of processing of a pattern, because of the necessity for material having high optical transmissivity in association with an increase in transmissivity or because of the necessity for material having high optical transmissivity in view of a conventional wavelength, in association with shortening of wavelength of the exposure light source. A multilayer-type halftone phase shifter section of two layers or more has an advantage of easy selection of material which enables control of a phase difference and transmissivity by means of a multilayer film or by combination of two films, as well as an advantage of the ability to select, as a lower layer, material which plays a role of an upper etch stopper layer. However, when the phase-shift mask is formed into a multilayer, a difference in etching characteristic between the layers presents a problem of difficulty in effecting highly-accurate CD (critical-dimension) control operation.
Further, a manufactured phase-shift mask must diminish reflectivity of exposure light to a certain extent. In a process for inspecting the appearance of a pattern, light having a wavelength longer than the wavelength of the exposure light is used as inspection light, and inspection is performed through use of a transmission-type optical inspection system (e.g., KLA 300 series). If transmissivity is higher than the inspection wavelength [when, e.g., an exposure wavelength corresponds to the wavelength of an KrF excimer laser (248 nm), an inspection wavelength is 488 nm or 364 nm] (for example, by 40% or more), inspection becomes difficult to perform. In particular, in association with shortening of an exposure wavelength, a halftone phase shifter section having a high optical transmissivity becomes required. However, material having a high optical transmissivity has such a tendency that the rate of increase toward a longer wavelength becomes greater. The halftone phase shifter of single layer becomes more difficult to reduce the optical transmissivity to the wavelength of inspection light to a predetermined range. Moreover, in relation to the optical inspection system, an inspection method using transmitted light and reflected light has been newly developed. The transmissivity achieved at the inspection wavelength when an inspection is performed according to this method may be slightly higher than that achieved when an inspection is performed through use of only transmitted light (e.g., by 50 to 60%). However, control must be performed such that the reflectivity achieved at the inspection wavelength becomes different from that of the transparent substrate by a certain extent (e.g., by 3% or more). More specifically, in reality, a request for controlling exposure light and a reflectivity at a wavelength differing from that of the exposure light (i.e., a wavelength slightly longer than that of the exposure light) also becomes more rigorous.
When a so-called tri-tone mask having a light transmission section, a semi-optical transmission section, and a opaque section provided on the semi-optical transmission section is inspected, reflection contrast must exist between the light transmission section/the semi-optical transmission section, between the semi-optical transmission section/the opaque section, and between the light transmission section/the opaque section. More specifically, when light reflectivity achieved at the light transmission section at the wavelength of the light source is taken as R1; light reflectivity achieved at the semi-optical transmission section at the wavelength of the light source is taken as R2; and light reflectivity achieved at the opaque section at the wavelength of the light source is taken as R3, R1<R2<R3 must stand.
A photomask is required to suppress reflectivity to exposure light to a certain extent. Among various evaluation and measurement instruments employed in a mask process, an apparatus for inspecting imperfections and extraneous matter in a mask blank and a mask and an apparatus for measuring flatness and stress employ a reflection optical system. Therefore, there also exists demand for the apparatus to have reflectivity which enables detection and measurement operation. In various pieces of evaluation and measurement apparatus employed in the mask process, the wavelength of the light source changes from one evaluation and measurement apparatus to another. Further, development of an inspection wavelength used for particle or defect inspection is pursued with a view toward employing a shorter wavelength. Therefore, the inspection wavelength changes according to whether the inspection apparatus is new or old. Examples of typical wavelengths of the light sources are as follows: wavelengths of light sources employed for particle or defect inspection of a blank are 488 nm, 364 nm; wavelengths of light sources employed for particle or defect inspection of a mask are 257 nm, 266 nm, and 364 nm; and wavelength of light sources employed for inspecting flatness and stress is 633 nm. Light sources in a wide wavelength range from a vacuum ultraviolet range to a visible range can be said to be employed. In reality, available wavelengths change from one user to another.
The surface reflectivity of a phase shifter film employed in a halftone-type phase-shift mask blank (mask) varies with reference to a wavelength. Simultaneous achievement of reflectivity within desired ranges at a plurality of wavelengths is difficult, and eventually leads to deterioration of precision in some steps of an evaluation process.
In particular, a phase shifter film of a two-layer-type halftone phase-shift mask blank (mask) having an upper phase adjustment layer and a lower transmissivity adjustment layer is liable to exhibit great variations in reflectivity, because the reflectivity of the phase shifter film plots a curve of interference potential (reflection spectrum) with reference to a wavelength, for reasons of interference arising between the light reflected from the lower layer and the light reflected from the upper layer. Difficulty is encountered in achieving a desired reflectivity over the above-mentioned wide wavelength range.
By means of combined use of the phase-shift mask, shortening of wavelength of the exposure light has been pursued. Use of an ArF excimer laser beam (193 nm) originating from argon fluoride (ArF) as shorter-wavelength light has been under review in recent years. Further, use of fluorine (F2) excimer laser beam (157 nm) as light having a much shorter wavelength has also been put forth.
In association with shortening of wavelength of the light, a wavelength band to be employed is said to shift to a deep ultraviolet range and further to a vacuum ultraviolet range. In a corresponding phase-shift mask and a corresponding phase-shift mask blank, as the exposure light approaches an ultraviolet range, control of desired transmissivity becomes difficult, because many substances are susceptible to a considerable increase in the degree of light absorption within a wavelength range shorter than 250 nm, as compared with a case where the exposure light approaches from a visible range to a near ultraviolet range. In connection with setting of transmissivity of a phase shifter, in the case of, e.g., a halftone-type phase-shift mask, it is considered desirable to have the ability to control the transmissivity of exposure light from within a range from 4% to 20% at the thickness of the phase shifter film that shifts the phase of the exposure light through a predetermined angle, depending on the sensitivity of a resist and the type of a mask (e.g., a transmission type or a halftone type), both being used in transferring a pattern.
In addition to control of transmissivity at the wavelength of exposure light in use, applicability of the invention to various inspections or alignment light sources, which are used in a process for manufacturing a blank, a process for manufacturing a mask, and a process for transferring a wafer, is also important. Light whose wavelength is longer than that of exposure light is usually employed for the inspections and the alignment light source. A wavelength close to an exposure wavelength is employed in apparatus of one or two generations prior. For instance, the appearance of a pattern is inspected through use of a transmission-type imperfection inspection apparatus (e.g., an KLA 300 series) at the wavelength of inspection light. In a case where the inspection wavelength [e.g., when the wavelength of the exposure light corresponds to the wavelength of KrF excimer laser (248 nm), the inspection wavelength assumes a 488 nm or 364 nm] is excessive with respect to transmissivity (by, for example, 40% or more), difficulty in inspection arises. In many materials, as a wavelength becomes longer, transmissivity of the material becomes higher. Hence, if an attempt is made to set low the transmissivity of desired light whose wavelength is longer than the wavelength of the exposure light, such as the inspection wavelength during designing of the phase shifter, there arises a problem of occurrence of a decrease in transmissivity with respect to the exposure light.
In the meantime, in association with miniaturization of an LSI pattern, the wavelength of the exposure light source (the wavelength of exposure light) is expected to become shorter from the current wavelength corresponding to a KrF excimer laser (248 nm) to the wavelength of an ArF excimer laser (193 nm), and further to that corresponding to an F2 excimer laser (157 nm). The film of the current mainstream halftone-type phase-shift mask is designed such that transmissivity of the exposure light of a halftone phase-shifter section assumes 6% or thereabouts. With a view toward attaining higher resolution, a halftone-type phase-shift mask having higher transmissivity is on its way to being requested. In the future, transmissivity of 15% or more is said to be required. In association with shortening of the wavelength of the exposure light source or an increase in transmissivity, the range of choices of materials of the halftone phase-shifter section, the materials satisfying predetermined transmissivity and the amount of phase shift, tends to become smaller. Moreover, etch selectivity to a quartz substrate becomes lower at the time of processing of a pattern, because of the necessity for material having high optical transmissivity in association with an increase in transmissivity or because of the necessity for material having high optical transmissivity in view of a conventional wavelength, in association with shortening of wavelength of the exposure light source.
Shortening of the wavelength of the exposure light and an increase in transmissivity also pose difficulty in developing and manufacturing a photomask. Responsible drawbacks will now be described.
First, in many solid materials, the degree of light absorption becomes greater as the wavelength becomes shorter. When a light transmission film material and a semi-optical transmission film material, both being used for a KrF excimer laser or an ArF excimer laser, are formed so as to assume a thickness which provides a predetermined phase angle, transmissivity of the materials approaches zero. Further, a high degree of absorption of exposure light means that the film forming the phase shifter section is correspondingly more susceptible to damage inflicted by the exposure light. Here, damage means a change in optical characteristic (i.e., transmissivity or refractive index) of the film, a change in the thickness of the film, and deterioration of the film, all stemming from imperfections which arise in a film constituting a phase shifter section by absorption of exposure light or to a split of coupling.
In addition, etch selectivity to the phase shifter film—which affects machining precision—and resistance to an oxide or alkaline used in a cleaning step during a manufacturing process must be considered in selection of film material used for forming a phase shifter section.